Seawater has conventionally been desalinated by filtration with a reverse osmosis membrane (also referred to as an “RO membrance”). In seawater desalination, a seafloor infiltration water intake method for taking in seawater that has naturally passed through a sand filtration layer of the seafloor is also known. With the seafloor infiltration water intake method, however, an amount of seawater intake may be reduced due to clogging substances such as silt getting caught in the surface layer of the sand filtration layer. In view of this, Japanese Patent Application Laid-Open No. 2013-86058 proposes a cleaning apparatus for removing clogging substances that have accumulated or become trapped in the surface layer of the sand filtration layer.
With seawater desalination using a reverse osmosis membrane, there is the problem of biofouling in which the membrance is clogged due to the growth of microorganisms such as bacteria. In order to suppress biofouling on the reverse osmosis membrane, fine filtration is preferably performed as a pretreatment on seawater that will be conveyed to the reverse osmosis membrane. In this case, it is conceivable to perform the pretreatment by using an ultrafiltration membrane (also referred to as an “UF membrance”) having a small molecular weight cut-off. However, as the molecular weight cut-off of the ultrafiltration membrane decreases, higher pressure needs to be applied to the seawater in order to ensure that a certain amount of seawater will pass through the ultrafiltration membrane, and energy consumption thus increases. In addition, the degree of separation (filtration) required for the ultrafiltration membrane differs according to the condition of seawater conveyed to the ultrafiltration membrane.